[Paper Review] Chemodynamics Of The Milky Way I. The First Year Of APOGEE Data

F. Anders, C. Chiappini+39 more|arXiv (Cornell University)|Nov 18, 2013

Stellar, planetary, and galactic studies108 references113 citations

TL;DR

This paper presents the first comprehensive chemo-dynamical analysis of the Milky Way's disc using the first year of APOGEE data, leveraging high-resolution near-infrared spectroscopy to map metallicity and [α/Fe] distributions across 6–11 kpc in Galactocentric radius and 0–3 kpc in vertical distance. It confirms a radial metallicity gradient inversion at high z, a gap in the [α/Fe] vs. [Fe/H] diagram, and a shorter scale-length for the thick disc, significantly expanding the volume of high-quality chemodynamical data beyond previous surveys.

ABSTRACT

We investigate the chemo-kinematic properties of the Milky Way disc by exploring the first year of data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), and compare our results to smaller optical high-resolution samples in the literature, as well as results from lower resolution surveys such as GCS, SEGUE and RAVE. We start by selecting a high-quality sample in terms of chemistry ($\sim$ 20.000 stars) and, after computing distances and orbital parameters for this sample, we employ a number of useful subsets to formulate constraints on Galactic chemical and chemodynamical evolution processes in the Solar neighbourhood and beyond (e.g., metallicity distributions -- MDFs, [$α$/Fe] vs. [Fe/H] diagrams, and abundance gradients). Our red giant sample spans distances as large as 10 kpc from the Sun. We find remarkable agreement between the recently published local (d $

Motivation & Objective

• To investigate the chemo-kinematic properties of the Milky Way disc using the first year of APOGEE data.
• To compare APOGEE results with smaller optical high-resolution samples and lower-resolution surveys like RAVE and GCS.
• To constrain Galactic chemical and chemodynamical evolution processes using abundance gradients and metallicity distributions.
• To assess the impact of survey selection functions on observed gradients and abundance patterns.

Proposed method

• Selection of a high-quality (HQ) stellar sample based on chemical and kinematical criteria, including radial velocities, proper motions, and distance estimates.
• Computation of orbital parameters (e.g., zmax, Rg) using phase-space information and Galactic potential models.
• Measurement of radial gradients in [Fe/H] and [α/Fe] across 6 < R < 11 kpc and 0 < z < 3 kpc, using binned data in Galactocentric radius and vertical height.
• Comparison of metallicity distribution functions (MDFs) and [α/Fe] vs. [Fe/H] diagrams between APOGEE and external samples (HARPS, GCS, RAVE).
• Use of bootstrap resampling to estimate 1σ uncertainties in gradient measurements.
• Initial testing of the TRILEGAL population synthesis code to model survey selection functions and validate observed distributions.

Experimental results

Research questions

• RQ1 How do the metallicity and [α/Fe] distributions in the Milky Way disc compare between APOGEE and high-resolution optical samples like HARPS?
• RQ2 What is the radial gradient of [Fe/H] and [α/Fe] in the disc, and how does it vary with vertical distance from the plane?
• RQ3 Is the observed 'gap' in the [α/Fe] vs. [Fe/H] diagram a real feature or a selection effect?
• RQ4 What do the abundance gradients reveal about the inside-out formation of the Galactic disc?
• RQ5 How do selection functions in APOGEE and RAVE affect the observed chemo-dynamical patterns?

Key findings

• The local MDF (d < 1 kpc) from APOGEE's HQ sample peaks slightly below solar metallicity and shows excellent agreement with the HARPS FGK dwarf sample, with a tail extending to [Fe/H] = −1.
• The APOGEE sample confirms the existence of a gap in the [α/Fe] vs. [Fe/H] diagram, indicating a bimodal distribution of α-element enhancement, and rules out selection effects as the primary cause.
• High-[α/Fe] stars are rare in the outer disc (R > 10 kpc), supporting a shorter scale-length for the thick disc compared to the thin disc, consistent with prior literature.
• A radial metallicity gradient of +0.052 ± 0.004 dex/kpc is measured for stars with 1.5 ≤ zmax < 3.0 kpc, indicating a positive gradient at high Galactic heights, in agreement with RAVE and GCS data.
• A radial [α/Fe] gradient of −0.031 ± 0.001 dex/kpc is found for stars with 1.5 ≤ zmax < 3.0 kpc, showing a negative gradient, consistent with RAVE and GCS results.
• The [α/Fe] distribution at high z shows no low-[α/Fe] stars, indicating minimal flaring of the thin disc in the observed radial range, and suggesting robustness of the orbital parameter estimates at high z.